1. Field of the Invention
The present invention relates to an antenna switching circuit for a diversity receiving system and a branching circuit with a signal attenuation operation being suitably provided within the antenna switching circuit.
2. Description of Background Information
A diversity receiving system is a type of radio reception in which two or more antennas are used for reducing adverse effects of the level change of an antenna output signal which might be experienced with a single antenna system.
In the field of diversity receiving, it is common to use an antenna switching circuit which receives RF signals from a plurality of antennas and transmits RF signals from one antenna to a receiver connected thereto. Among conventional antenna switching circuits, a drawback was that more than one output terminals were required for preventing a level drop if RF signals of different frequency bands such as the VHF band and the MF band are to be treated by an antenna switching circuit. Therefore, it has been desired to present an antenna switching circuit which can provide RF signals of different frequency bands through a single output terminal.
On the other hand, a branching circuit is generally used for separating RF signals of different frequency bands and deriving the RF signals separately, in such an occasion that a single antenna is commonly used for receiving the RF signals of the MF band (AM broadcasting band) and the VHF band (FM broadcasting band). Further, there is generally provided, on the receiver side, an AGC (Automatic Gain Control) circuit for attenuating the level of the input RF signal in an RF amplification stage, so as to avoid signal distortion when an RF signal of very high intensity is received. Under these circumstances, it is advantageous, if possible, to attenuate the level of RF signals in one of different freuquency bands within a branching circuit, in such a case the RF signals of different two frequency bands, such as the AM and FM broadcasting bands, are separated in a branching circuit and supplied to a receiver.